You’ve turned 65 and exited middle age. What are the chances you’ll develop dementia in the years ahead?

New research about “cognitive life expectancy” — how long older adults live with good versus declining brain health — shows that after age 65 men and women spend more than a dozen years in good cognitive health, on average. And, over the past decade, that time span has been expanding.

By contrast, cognitive challenges arise in a more compressed time frame in later life, with mild cognitive impairment (problems with memory, decisionmaking or thinking skills) lasting about four years, on average, and dementia (Alzheimer’s disease or other related conditions) occurring over 1 ½ to two years.

Even when these conditions surface, many seniors retain an overall sense of well-being, new research said. “The majority of cognitively impaired years are happy ones, not unhappy ones,” said Anthony Bardo, a co-author of that study and assistant professor of sociology at the University of Kentucky-Lexington.

Recent research finds that:

Most seniors don’t have cognitive impairment or dementia. Of Americans 65 and older, about 20 to 25 percent have mild cognitive impairment while about 10 percent have dementia, said Dr. Kenneth Langa, a professor of medicine at the University of Michigan. Risks rise with advanced age, and the portion of the population affected is significantly higher for people older than 85.

Langa’s research shows that the prevalence of dementia has fallen in the U.S. — a trend observed in developed countries across the globe. A study from researchers at the Rand Corp. and the National Bureau of Economic Research finds that 10.5 percent of U.S. adults 65 and older had dementia in 2012, compared with 12 percent in 2000.

Because the population of older adults is expanding, the number of people affected by dementia is increasing nonetheless: an estimated 4.5 million in 2012, compared with 4.1 million in 2000.

Improvements in education and nutrition, better control of hypertension and cholesterol, cognitively demanding jobs in middle age, and social engagement in later life may all contribute to this expanded period of good brain health, the study noted.

College graduates can expect to spend more than 80 percent of their lifetime after age 65 with good cognition, said researchers at the University of Southern California and the University of Texas at Austin. For people who didn’t complete high school, that drops to less than 50 percent.